Carrier tapes are of ever increasing importance in assembly operations in numerous industries including, in particular, the electronic and pharmaceutical industries. The tapes are used, for example, to transport miniature and subminiature electronic components from a component manufacturer to a customer's assembly station where automatic equipment functions to remove the component from the carrier tape and mount the component to a circuit board or the like. The carrier tape (or transfer tape, as it is sometimes referred to) may also function to bring different components to an assembly station in proper order for sequential assembly.
Such carrier tapes are often in the form of a base strip of plastic or paper which is usually covered by a cover strip. The base strip is formed with a plurality of cavities or pockets for the components and the cover strip is secured to the base strip to thereby prevent the components from prematurely falling out of the cavities. In the past the base strip and cover strip were most commonly held in place by an adhesive, heat bound or spaced snap-type fasteners. Typical examples of prior art carrier tapes are disclosed in U.S. Pat. Nos. 3,465,874; 3,650,430; 3,700,019; 3,894,896; 3,910,410; and 4,298,120. The U.S. Pat. Nos. 4,708,245 and 4,733,778 disclose a continuous mechanical interlock between the base strip and cover strip which obviates many of the problems associated with the use of the various prior art methods of securing the base strip and cover strip.
To function properly, a carrier tape must securely convey its cargo as required and then, at the proper location, be capable of being quickly and positively opened so that the component may be easily removed for assembly purposes. The tape should afford a degree of protection for the sensitive components to be conveyed therein with respect to contamination, electro-static discharge, temperature change and mechanical shock. In addition, the seal or bond between the tape components must be releasable to a known and consistent value at the unloading point to insure that only the required length of base tape will be exposed. Prior art tapes utilizing adhesive or heat bonding techniques present a risk of residue contamination. Further, heat bonding requires the use of relatively high temperatures which may be harmful to the components to be carried. In addition, the use of static-dissipative or conductive additives to prevent or dissipate the build-up of electrostatic charge tends to interfere with the bonding characteristics of adhesives. The previously mentioned application Ser. No. 046,259 discloses an improved carrier tape construction in which the cover strip and base strip are uniformly and consistently secured to one another yet which can readily and positively be separated from one another.
To this end, the carrier tape of the previously mentioned application comprises an elongated base strip and an elongated cover strip both formed of a plastic film material. The base strip is provided with longitudinally spaced cavities for components to be transported therein as well as a series of spaced, longitudinally extending sprocket drive holes for advancing the tape. The cover tape has sidewalls extending downwardly and terminating in upwardly directed barbs or profiles. The spacing between the sidewalls is substantially equal to the width of the base strip. A raised, longitudinally extending hump is provided in the top face of the cover strip aligned with the sprocket holes when the tape is assembled. The hump defines a living hinge for the adjacent surfaces of the cover strip top face. Accordingly, when the hump is depressed the sidewalls of the cover strip move outwardly so as to clear the edges of the base strip and permit removal of the cover strip. Conversely, when the hump is released, the sidewalls return to their relaxed position securely interlocking with the base strip edges. While this construction avoids many of the problems associated with carrier tapes previously available, it introduces other problems. Namely, when such carrier tapes are loaded and shipped, they are commonly wound on a reel or coiled. If the coil is wound too tightly, a force is exerted on the hump of the inner convolutions tending to cause the sidewalls of the cover to move laterally outwardly. In an extreme situation, the cover strip sidewalls may move outwardly sufficiently to disengage from the base strip thereby exposing the component cavities and raising the possibility of some components falling out of their cavities. This, obviously, is not a tolerable situation.
In view of the above, it is the principal object of the present invention to provide an improved carrier tape construction which avoids the use of adhesive and chemical bonding and in which the cover strip and base strip are uniformly and consistently secured to one another even when the carrier tape is wound in a tight coil.
A further object is to provide such a carrier tape wherein the cover strip and base strip may readily and positively be separated from each other during loading and unloading operations.
Still another object is to provide such a carrier tape which may readily be used with existing equipment with little or no modification required.
Yet another object is to provide such a carrier tape which may readily be manually opened and closed so as to facilitate the inspection and replacement of the components carried therein.
A still further object is to provide a method of manufacturing the improved carrier tape of the present invention in a reliable and economic manner.